Control of audio frequency transmission



y 1940- H. BARTELs ET AL ,201,022

CONTROL OF AUDIO FREQUENCY TRANSMISSION Filed Nov. 17, 1936 In E2 Icon/moan) M/p 4107. 1. #752 mm mm our/ ur ATTORNEY Patented May 14, 1940UNITED STATES CONTROL OF AUDIO FREQUENCY TRANSMISSION Hans Bartels andGiinther Ulbricht, Berlin, Germany,

assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b.11., Berlin, Germany, a corporation of Germany Application November 17,1936, Serial No. 111,236 In Germany December 10, 1935 3 Claims.

"This invention relates to a circuit arrangement for controlling theintensity range of audiofrequency transmission stations.

In arrangements for controlling the intensity 5 range in audio-frequencytransmission stations (microphone modulation arrangements, talking filmarrangements, transmitter modulation a1- rangements and the like) it wasfound to be of advantage that various intensity range control curves forthe amplifier can be adjusted to, since various optimum control curvesare assigned to difierent conditions of recording. The most favorabledegree of control is dependent to a greater or lesserdegree on the typeof programs. Therefore, it is of advantage to so design the controlapparatus that various control curves can be selectively obtained. Onaccount of the logarithmic volume response of the human ear, variationin the intensity range of electroacoustic performances between equallimit values will be least noticeable if the proportion between theamplitudes of the voltages ahead and after the control member isadjusted to a logarithmic curve, when plotted on a linear scale, or alinear curve in a double-logarithmic scale. That is, if the curve of thecontrolled voltage follows a logarithmic variation in the linear scaleor a linear course in a double logarithmic scale.

This is accomplished in accordance with the invention in that in thetransmission path of the auxiliary control circuit a voltage dividerformed of resistance depending on current such as rectifiers isprovided, and from which the voltage passing to the control member istapped in accordance with the desired control curve.

Since the amplification path of the main amplifier is calculated for themaximum voltage, it is advisable also when reducing the auxiliaryvoltage by the voltage divider circuit of the nonlinear resistance, thatcare be taken that the main amplifier is utilized always up to the limitof its output capacity. In certain cases it may be of advantage toadjust to the average volume This is done in such manner thatsimultaneously with the variation of the tapping at the rectifiervoltagedivider a volume control device in the amplifier, such as apotentiometer located between two amplifier stages, is correspondingly50 varied.

Figures la, 12? and 1c are curves illustrating our improvement whileFigure 2 is a fragmentary circuit diagram embodying our invention.

The methods of control according to my inven- 5 tion are elucidated bythe Figures la, 1b, and 1c.

' stance.

Fig. 1a shows in logarithmic scale the alternating output potential asfunction of the alternating input potential. The line I inclined by 45?towards the abscissa axis corresponds to the uncontrolled state. Theline II indicates for in-'-'5 stance the control 1:3.- The line IIIshows for instance the control 1:6, the line IV for instance the control1:10. As seen from the diagram, the maximum alternating output potentialEa available is not fully utilized. Only the value ne is utilized.

In order to assure a more favorable operation of the amplifier,therefore, simultaneously with the setting of the tap point at thenon-linear resistances of the auxiliary circuit, also a volume 15control device in the transmission path is adjusted so that degrees ofcontrol according to Fig. 1b are obtained. The uncontrolled state isagain represented by theline I. The lines II, III, and IV againrepresent the corresponding conditions according to Fig. 1a. The line IVshows, for-instance, the control ratio 1:10. In certain cases, it isdesirable to adjust to an average volume instead of setting the maximumvolume. In this case, the volume control and the voltage divider formedof the non-linear resistances in the auxiliary circuit would have to bedimensioned in such manner as indicated in the diagram IQ, that is asregards volume there is to' be maintained stationary a point within therange between the minimum input potential and the maximum inputpotential.

Fig. 2 shows an example of embodiment of. the idea of the invention,indicating however, only the auxiliary control circuit in detail. Theimpulses derived from the main transmission path at the input ofamplifier MV are applied to the auxiliary amplifier HV and are rectifiedin the control rectifier Rgl. The detected voltage is applied to thevoltage divider S29 formed of the non-linear resistances such asdetector for in- The taps connected to the control grid of thecontrolled amplifier CV are designated by I, 2, 3 and 4. When tap A isin contact with terminal point I the auxiliary amplifier H; has noeffect on the signals. In accordance as to whether the tap A contactsthe points 2, 3 or 4 a direct potential having a higher or lower valuewill be applied to the control grid, and thereby the desired switchingof the control curves obtained.

As shown in Figure 2 the volume control V0 is conveniently insertedbetween amplifier stages MV and CV in the main signal channel. When tapA is in contact with one of the points identified by numerals I 2, 3 and4 the amplifier operates along one of the characteristic curves shown inFigures 1a, 1b, 10 having the corresponding Roman numerals.

As already explained with reference to Figs. 1alc, it may also be ofadvantage to couple automatically the tap point A with a volume controlVC inserted in the amplification train, so that at changing of thecontrol curve, also a change in the average transmission level will besimultaneously obtained.

What is claimed is:

1. In combination, an amplifier circuit having input terminals, anauxiliary control channel connected to said input terminals, a rectifierfor rectifying signals in said auxiliary channel,

a tapped voltage divider comprising a non-linear resistance connected tosaid rectifier, means for applying selected portions of thevoltage'developed across the said divider to a control circuit of saidamplifier, a volume control in said amplifier said volume control andsaid last mentioned means being mechanically coupled together.

2. In combination, an amplifier circuit having input terminals, anauxiliary control channel connected to said input terminals, a secondamplifier in said auxiliary channel, a rectifier for rectifying signalsto said second amplifier, a tapped voltage divider including anon-linear resistance connected to said rectifier, means for applyingselected portions of the voltage developed across said divider to acontrolled circuit of the first named amplifier, a volume control insaid first named amplifier, said volume control and said last mentionedmeans being mechanically coupled together.

3. In combination, an amplifier circuit having input terminals and anoutput, said input terminals being adapted to have varying amplitudesignals applied thereto, an auxiliary control channel connected to saidinput terminals, a rectifier for rectifying signals in said auxiliarychannel, a tapped voltage divider including a non-linear resistanceconnected to said rectifier, means for so applying selected portions ofthe voltage developed across said divider to a control circuit of saidamplifier that the amplitude of signals in said output varieslogarithmically with respect to the amplitude of signals at said inputterminals.

HANS BARTELS. GfTNTI-IER ULBRICHT.

